Comment by edgineer

IMO the top motivation and method of using this tool both is when you notice videos in a playlist going dark.

On desktop, you'd view the full playlist and notice "Unavailable videos are hidden" at the top. Click the kebab menu and "Show unavailable videos" and the list will populate grey-icon videos which do still link to the broken video so you can get the ID.

But videos will also be broken in a way that this method does not reveal (it will only show private and deleted videos). Copystruck or georestricted videos you'll only find out once you attempt to view. Use a YouTube playlist downloader to quickly see which those are.

And then there will be videos you know existed but aren't in a playlist--I have relied on googling around for old forum and blog posts that include the link in order to get the original ID, if I didn't click on a broken link in the first place.